1. Field of the Invention
The present invention relates to the use of a coherent optical beam modulating system that utilizes continuously tunable stepped-well quantum well infrared modulators.
2. Description of Related Art
Many methods exist that accomplish the deflection and steering of optical signals. The most common methods employ a mechanical means to accomplish the directional modification of an optical beam or spatial patterns of illumination produced by lasers. Spatial light modulators of the prior art have been more recently replaced with quantum well devices. The quantum well devices used for light modulation provide smaller spatial light modulators where diffraction effects dominate, have fast response times and can be made lithographically using standard fabrication equipment. Beam steering devices have implemented the optical modulators where the phase of lightwaves is altered by illumination of semiconductors. The spatial light modulators function by optically inducing changes in the semiconductor material which affect an adjacent layer of electro-optic material which in turn affects EM wave propagating through the electro-optic material.
Spatial light modulators include the use of phased array antennas that transmit EM waves in a particular direction in the microwave region without moving parts. The phased array antennas electronically change the phases of the signals radiating from each element. Accordingly, electronic beam steering provides a faster and more agile steering method as opposed to mechanical beam steering and facilitates multiple target tracking by forming several beam lobes and nulls. The use of phased array antennas possesses some drawbacks such as diminished control of each element that requires the use of a power splitting network which increases cost and size. Furthermore, large arrays require significant computations in order to calculate array phase distribution.
U.S. patent application Ser. No. 10/145,073 to Sundaram (Sundaram), incorporated by reference, describes an electronically steered laser beam that uses an absorption modulator for electronic beam steering. The device of the Sundaram application operates as a mirror in the reflection mode and an optical medium in the transmitter mode. An integrated circuit or chip reflects the laser beam in a reflection mode by varying the voltage of the chip. In the alternative transmitting mode, the chip transmits the laser beam according to the applied voltage. Sundaram describes the use of an optical modulator array formed on a substrate with a quantum well doped with electrons. Discrete voltage signals are applied to the modulator to control an exit angle of one or more exit beams from the modulator. The modulator operates by using intersubband transitions in the 3–5 micron range for countermeasures applications.
Intersubband transitions have sharp absorption lines with spectral widths Δλ/λ (Full-width-half-maximum/Peak-wavelength) as low as 8%. On the other hand, interband absorption has undesirably wide spectral width and in the 3–5 micron wavelength range is hampered by the lack of a semiconductor system with the properties required to achieve voltage-tunable absorption. Quantum well absorption of 3–5 micron wavelength is possible in one embodiment using lattice-matched InGaAs/InAlAs quantum wells on InP. It is also possible to have proper operational characteristics in another embodiment using strained InGaAs/GaAs/AlGaAs quantum wells on GaAs substrate.
The ability to change the quantum well absorption with applied voltage is possible by charge transfer to and from the quantum wells capacitively. An alternate method to tune quantum well absorption is called the Stark effect and relies on tuning the energy levels in the quantum well with voltage, thereby changing the absorption peak, which is equal to the separation between the two lowest energy levels. This effect is generally very small, particularly at non-cryogenic temperatures, such as room temperature, with the resulting absorption modulation too small to make a practical device.
An array of absorption modulators may be formed on a substrate wherein the absorption spectrum changes with the voltage bias. In some instances, the absorbers are inserted in the resonant cavity. The absorption changes with the voltage per pixel; the phase changes with the voltage per pixel; the amplitude changes with the voltage per pixel (ideally zero); and therefore, the beam angle changes with an array voltage pattern.
In regard to standard quantum well infrared photodetectors (QWIP), QWIPs rely on a symmetrical structure design involving a series of quantum wells and barrier layers grown sequentially. The center of symmetry lies at the center of each quantum well, therefore such structures should in theory respond identically to a bias of either polarity. In particular, their spectral response should have at best a second-order dependence on the applied bias. FIGS. 1A and 1B depict experiments that indeed confirm that conventional QWIP's have a very weak dependence of the peak wavelength on applied bias, at least over the few volts afforded by normal read-out integrated circuits (ROIC).
U.S. Pat. No. 6,353,624 to Pelekanos et al. (Pelekanos) relates to a semiconductor laser with a tunable gain spectrum. The tuning principle of Pelekanos is based on a modulation by an electro-optical effect such as the quantum confined Stark effect of the gain spectrum of the diode during the emission of the laser radiation. The wavelength of the laser radiation is directly controlled by the current injected into the laser diode.
U.S. Pat. No. 4,903,101 to Maserjian (Maserjian) relates to an infrared detector characterized by photon-assisted resonant tunneling between adjacent quantum wells formed in a semiconductor structure, wherein the resonance is electrically tunable over a wide band of wavelengths. The quantum well structure of Maserjian comprises at least one quantum well sandwiched by two barrier layers and formed in an intrinsic layer comprising an III–V semiconducting material.
In order to create a quantum well infrared modulator (QWIM), one must break the bilateral symmetry of the quantum well structure by incorporating some irregularity into the design of the unit cell. The asymmetry allows a first-order dependence of the peak wavelength on applied bias. QWIMs are created in two fashions. The first approach involves designing a unit cell composed of two separate but closely spaced quantum wells, each with its own distinct spectral response. The energy bands tilt by means of an applied bias in such a way that the electrons transfer from one well to the other, leading to discontinuous or bimodal spectral tunability. The second approach involves an asymmetrically stepped quantum well in which the separation between the ground state (E1) and the first excited state (E2) changes linearly with applied bias.